Our District

Rain gardens and permeable paving

Spring is here! And with it, a lot of rain.

Heavy downpours, like those we’ve seen recently, test the limits of paved areas and paths.

Are paths in the right places, or are we cutting a muddy track across the lawn? Are they at the right level? Can they handle large amounts of rain? Paving that’s too low means puddles in all the wrong places. And if driveways are turning into waterways, it may be time for a drainage rethink. 

After a heavy downpour, look around your garden. It becomes obvious where the natural hollows, low points and watercourses are. If these are on a well-trodden path (because of ground compaction) you may need to raise the paving level, or create better drainage beside it – for example by digging trenches.

Get to know your property, and you can turn pooling and puddles into raingardens, swales or watercourses. 

On Slopes: Rain always follows the path of least resistance, and can scour out mulch and topsoil, carrying it into the stormwater system. Slowing water down is the key to letting it sink in, rather than run off.

  • Creating swales (ditches that run sideways, across the contour of the slope) has several advantages. Swales slow the flow of water and act as pockets to collect topsoil and mulch. Plants growing around these pockets benefit from the extra groundwater in dry weather too.  See more information on how to build a swale here.
  • Low walls of ponga or logs can also be useful to terrace slopes, creating planting pockets.
  • Rocks, stones and logs can be used to create a ‘dry riverbed’ that becomes a temporary stream in wet weather, perhaps flowing into a raingarden. Plant ferns and other thirsty plants along the edges.

Raingardens: Areas where water pools in heavy rain can be turned into rain gardens. These mini-wetlands hold water for hours or 1-2 days after heavy rain, letting it gradually drain into the soil so it’s filtered before entering waterways. Roof stormwater can also be diverted into raingardens.

On sandy soil, make a depression lower than the surrounding area, with gently sloping sides. The size of the raingarden depends on the amount of water it will get. Plant it up with natural wetland plants like rushes, sedges and flaxes, or groundcovers like leptinella and gunnera. A rock or pebble mulch is good, as bark mulch can float away when the area fills with water.

On heavier soils, raingardens might need an underdrain and/or an overflow. For more information see here.

Plants as filters: The bush acts as a natural water holder and filter, releasing water slowly over time. Compare a valley of sodden bush with a valley of pasture – you’ll see far more soil erosion and dirtier streams in the farmland. We can’t all have bushclad properties, but even on a small section, plantings of shrubs and grasses hold and filter water better than paved areas or lawns.

Permeable paving: This means a driveway or path that lets some rain sink through. Permeability can be provided in different ways; by using a permeable surface like lime sand or gravel rather than hard paving; by allowing cracks between pavers; or by using specially-made water-permeable pavers. Weed matting and/or a compacted base course layer underneath will provide further stability and drainage and make your paving last longer.

For driveways, consider having concrete strips for the tyres rather than paving the whole area. There’s a great gallery of ideas on pinterest here.

The bigger picture: Where does water go when it leaves your property? Rain falling on hard surfaces- driveways, concrete, patios – can’t sink in, as it can on gardens or lawns. Unfortunately it often ends up dirtying our streams and waterways.  To reduce flooding on a larger scale and clean up Kapiti’s waterways, support local wetland restoration projects. For a list of contacts see here.

 

    

 

left: Lime sand makes an attractive, hardwearing and permeable surface between raised beds

right: Permeable ‘crazy' paving

PAVING MATERIALS

 

Pros

Good for

Cons

Rounded river pebbles

Attractive, natural surface; available free at yearly ‘gravel grab’.

Flat driveways, low traffic paths, open areas

Not stable on slopes, not great for those who need an even walking surface

Crazy paving or pavers with gaps between

Concrete or stone pavers with gaps between, letting water drain through. Can have plants, gravel or sand in the gaps.

Patios, low traffic paths

Weeds can pop up in the gaps –low plants like pratia (shade) or thyme (sun) can keep weeds out.

Lime sand

Attractive, freedraining surface

Paths, around raised beds, patio areas

Not cheap, will need topping up every few years

Gravel

Pea metal- fine gravel

Paths, open areas

Needs hoeing and topping up. Not ideal where leaves will fall.

Sawdust

Cheap, attractive, ‘knits’ into a firm surface

Paths among raised beds

May be shortlived

Permeable pavers

Specially made to hold/drain water – no extra drainage needed.

Patios, driveways, paved areas, paths

Not cheap, but should last a long time.